The Senghenydd Colliery Disaster, also known as the Senghenydd Explosion (Welsh: Tanchwa Senghennydd), occurred in Senghenydd,[1] near Caerphilly, Glamorgan, Wales, on 14 October 1913, killing 439 miners and one rescuer. It is the worst mining accident in the United Kingdom, and one of the most serious globally in terms of loss of life.[2] The explosion gained this distinction nearly half a century after the previous worst disaster – the Oaks explosion at Oaks Pit, in Barnsley, Yorkshire, on 12 December 1866, when 388 workers died in two separate explosions.[3]

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The demand for Welsh steam coal before World War I was enormous, driven by the huge increase in the export trade in Welsh coal from the 1840s. Coal output from British mines peaked in 1913, and there were a correspondingly large number of accidents around this time.

Universal Colliery was developed from 1891, and owned by William Thomas Lewis.[4] In 1901 an explosion at the colliery killed 81 men. The Mines Inspectorate was critical of Lewis for not improving safety. In 1911, Lewis was created 1st Baron Merthyr. In that same year, the new Mines Act required that all collieries were to be capable of reversing the air current ventilating the mine by the 1st January 1913. No work was undertaken at Senghenydd to implement this requirement and the Mines Inspectorate gave Lewis an extended deadline of September 1913 to complete the work, but again this deadline was missed.

On the morning of 14 October 1913, six weeks after the final mines inspectorate deadline (which had been missed) there were approximately 950 men working in Universal Colliery's two pits. Just after 8am an explosion ripped through the west side underground workings. The cause was probably a buildup of firedamp (methane) being ignited by electric sparking from equipment such as electric bell signalling gear. The initial explosion disturbed coal dust present on the floor, raising a cloud that then also ignited. The shock wave ahead of the explosion raised yet more coal dust, so that the explosion was effectively self-fueling. Those miners not killed immediately by the fire and explosion would have died quickly from afterdamp, the noxious gases formed by combustion. These include lethal quantities of carbon monoxide, which kills very quickly by combining preferentially with haemoglobin in the blood, resulting in suffocation by lack of oxygen or anoxia.

Survivors were extracted from the colliery with the last 18 miners rescued in the early hours of 15 October. The resulting funerals took over a month to complete. The mines manager was fined £24 for breaches of the mines safety code, whilst the owner William Thomas Lewis was fined £10.

Four memorials to the disaster are located in Senghenydd. The first is outside Nant-y-parc Primary School,[5] which is built on the site of the old mine. At St. Cenydd Comprehensive School is a list of names of those who died from the explosion, and they have a truck of coal as a memorial. There is also a memorial at the local pub, the Green Pint.

On 14 October 2013, on the centennial of the disaster, a Welsh national memorial to all mine disasters was unveiled at the former pit head. With co-funding raised by the Aber Valley Heritage Group and their patron Roy Noble, and matched by the Welsh Assembly Government, a bronze statue designed by sculptor Les Johnson FRBS, depicting a rescue worker coming to the aid of one of the survivors of the explosion, was unveiled by First Minister of WalesCarwyn Jones.[6]

A stage play based on the disaster, by journalist and broadcaster Margaret Coles, was first performed at the Sherman Theatre, Cardiff.[7]